The present invention generally relates to a laser system and more particularly to a laser system using ultrashort laser pulses with phase modulation.
Commercially practical femtosecond lasers have been unavailable until recently. For example, lasers which can generate 10 femtosecond or less laser pulse durations have traditionally been extremely expensive, required unrealistically high electrical energy consumption (for extensive cooling, by way of example) and depended on laser dyes that had to be replenished every month thereby leading to commercial impracticality. The efficiency of sub-10 femtosecond lasers was not practical until the year 2000 because of the prior need for dyes and flash lamps instead of YAG and Ti: Sapphire crystals pumped by light or laser emitting diodes.
Ultrashort pulses are prone to suffer phase distortions as they propagate through or reflect from optics because of their broad bandwidth. There have been recent experimental attempts to shape the phase of ultrashort pulses since shaped pulses have been shown to increase the yield of certain chemical reactions and multiphoton excitation.
Conventional pulse characterization is typically done by one of the following methods. Autocorrelation is a simple traditional method that yields only the pulse duration. Furthermore, frequency resolved optical gating (hereinafter “FROG”) is a known method which yields phase and amplitude following iterative analysis of the time-frequency data. Interferometric methods such as DOSPM and spectral phase interferometry (hereinafter “SPIDER”) yield phase and amplitude from frequency resolved Interferometric data; these are very complex and expensive but reliably provide the required information. Both FROG and SPIDER methods require some type of synchronous autocorrelation setup. In the case of the FROG method, autocorrelation is used to provide a time axis while a spectrometer provides the frequency domain information. In the case of the SPIDER method, the ultrashort pulse is split into three beams during autocorrelation; the pulse in one of the beams is stretched to provide the shear reference, while the other two pulses are cross-correlated with the stretched pulse at different times. The output is sent to a spectrometer where the interference in the signal is used to reconstruct the electric field. This extra synchronous autocorrelation step adds time and cost in addition to necessitating highly skilled operators. Limitations with prior devices and methods are discussed in R. Trebino et al., “Measuring Ultrashort Laser Pulses,” Optics & Photonics News 23 (June 2001). Moreover, the Grenouille method requires a setup consisting of a Fresnel biprism, a doubling cryst and lenses that need to be specifically chosen for a particular pulse duration and wavelength, making this method less flexible.
In accordance with the present invention, a laser system using ultrashort laser pulses is provided. In another aspect of the present invention, the system includes a laser, pulse shaper and detection device. A further aspect of the present invention employs a femtosecond laser and a spectrometer. Still another aspect of the present invention uses a laser beam pulse, a pulse shaper and a SHG crystal. In yet another aspect of the present invention, a multiphoton intrapulse interference phase scan (hereinafter “MIIPS”) system and method characterize the spectral phase of femtosecond laser pulses. In another aspect of the present invention, a system employs electromagnetic pulse shaping design to take advantage of multiphoton intrapulse interference. Fiber optic communication systems, photodynamic therapy and pulse characterization tests use the laser system with additional aspects of the present invention.
The laser system of the present invention is advantageous over conventional constructions since the MIIPS aspect of the present invention employs a single beam which is capable of retrieving the magnitude and sign of second and third order phase modulation directly, without iteration or inversion procedures. Thus, the MIIPS system is much easier to set up and use, thereby creating a much less expensive system which is more accurate than conventional systems and methods. Furthermore, the MIIPS system of the present invention avoids the inaccuracies of the prior FROG, SPIDER and DOSPM methods due to environmental effects such as wind, humidity and the like. The present invention MIIPS system utilizes the full bandwidth which works best with shorter laser beam pulses, such as femtosecond pulses; this is in contrast to the mere single frequency optimization of some convention devices. The present invention MIIPS system overcomes the traditional need for slower picosecond pulses for space-time correlation corrections due to inherent time delays created with prior synchronous use of multiple matched pulses, a first pump or fundamental pulse and another reference second harmonic pulse, caused by the pulse passage through a pulse shaping crystal. Additionally, the present invention advantageously uses one or more pre-stored comparison values for pulse signal decoding at a communications receiver such that the second reference pulse (and corresponding time delay correlation) are not necessary. The present invention also improves the encoding-decoding functionality of pulses by adding considerably more information to each pulse by obtaining the entire phase function directly from a phase scan. Intrapulse interferences of the present invention causes self separation (for example, inherent communication signal routing address differentiation) thereby allowing use of inexpensive receivers in an asynchronous manner, in other words, without the need for synchronous detection such as by traditional autocorrelation or interferometers. Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.